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Newsgroups: comp.lang.ada
Date: Tue, 29 Nov 2016 12:45:10 -0800 (PST)
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Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
From: Shark8 <onewingedshark@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:32517
Date: 2016-11-29T12:45:10-08:00
List-Id: <comp.lang.ada>

On Tuesday, November 29, 2016 at 10:53:59 AM UTC-7, Niklas Holsti wrote:
> On 16-11-29 01:49 , Shark8 wrote:
> > So, with Ada 2012 we gained some really nice possibilities
> > with the way to express constraints, the downside is that
> > there's now a fairly wide range of ways to express constraints on
> > types. Obviously these differences must be accounted for, but
> > the are functionally equivalent, for example:
> >
> > Subtype P0 is Natural range Natural'Succ(Natural'First)..Natural'Last;
> 
> This seems the same as Positive, to me.

It is.

> > Subtype P1 is Integer range 1..Integer'Last;
> 
> And this is the standard definition of Positive.

It is.

> > Subtype P2 is Integer
> >     with Static_Predicate => P2 in 1..Integer'Last
> >                           or else raise Constraint_Error;
> 
> Perhaps it was obvious to you, but this differs from Positive in that 
> you cannot write P2'First or similar (illegal by RM 3.2.4(26/3)), but 
> you can write Positive'First.

But insofar as a "type" (the collection of values and operations*) goes it is the same: its values have the same range, the operations* are the same, and it raises the same error when they are violated.


* Assuming we're considering attributes as something different than an 'operation' and excluding them.

> 
> > Subtype P3 is Integer with
> >   Static_Predicate  => P3 in 1..Integer'Last,
> >   Predicate_Failure => raise Constraint_Error;
> 
> Same difference as for P2, I think.
> 

So there *IS* something in common about all the above, that's what the thrust of the topic (an Ada IR) is getting at. Something like "can't use attributes like 'First" could be handled [implementation-wise] as something like:

-- Vastly oversimplified.
Type Subtype_Node( Accessible_Attributes : Boolean := True ) is record
  Name   : String_Holder;
  Parent : Type_Node;
  Values : -- value-set.
  -- or however we represent the constraints, errors, and messages.
enc;
end record;